问题描述

我有一个多线程应用程序，它大量使用 std::cout 进行日志记录而没有任何锁定.在这种情况下，如何轻松添加锁机制使 std::cout 成为线程安全的?

I have a multi-threaded application, which heavily uses std::cout for logging without any locking. In such a case, how can I easily add lock mechanism to make std::cout thread-safe?

我不想搜索每次出现的 std::cout 并添加一行锁定代码.太麻烦了.

I don't want to search for each occurrence of std::cout and add a line of locking code. That is too tedious.

有什么更好的做法吗?

推荐答案

注意:这个答案是 pre-C++20 所以它不使用 std::osyncstream 具有单独的缓冲，但使用锁代替.

我猜您可以实现自己的类，该类包装 cout 并将互斥体与其关联.那个新类的 operator << 会做三件事:

I guess you could implement your own class which wraps cout and associates a mutex with it. The operator << of that new class would do three things:

1. 为互斥锁创建一个锁，可能会阻塞其他线程
2. 执行输出，即对包装的流和传递的参数执行操作符 <<
3. 构造一个不同类的实例，将锁传递给那个

1. create a lock for the mutex, possibly blocking other threads
2. do the output, i.e. do the operator << for the wrapped stream and the passed argument
3. construct an instance of a different class, passing the lock to that

这个不同的类将把锁和委托操作符 << 保留到包装的流中.第二个类的析构函数最终会破坏锁并释放互斥锁.

This different class would keep the lock and delegate operator << to the wrapped stream. The destructor of that second class would eventually destroy the lock and release the mutex.

因此，您作为单个语句编写的任何输出，即作为 << 调用的单个序列，只要您的所有输出通过具有相同互斥量的对象，就会自动打印.

So any output you write as a single statement, i.e. as a single sequence of << invocations, will be printed atomically as long as all your output goes through that object with the same mutex.

让我们调用两个类 synchronized_ostream 和 locked_ostream.如果 sync_cout 是 synchronized_ostream 的一个实例，它包装了 std::cout，那么序列

Let's call the two classes synchronized_ostream and locked_ostream. If sync_cout is an instance of synchronized_ostream which wraps std::cout, then the sequence

sync_cout << "Hello, " << name << "!" << std::endl;

将导致以下操作:

1. synchronized_ostream::operator<<< 会获得锁
2. synchronized_ostream::operator<< 会将Hello,"的打印委托给 cout
3. operator<<(std::ostream&, const char*) 将打印Hello,"
4. synchronized_ostream::operator<< 会构造一个 locked_ostream 并将锁传递给那个
5. locked_ostream::operator<< 会将name 的打印委托给cout
6. operator<<(std::ostream&, std::string) 将打印名称
7. 同样的委托给 cout 发生在感叹号和结束线操纵器
8. locked_ostream 临时被破坏，锁被释放

1. synchronized_ostream::operator<< would aquire the lock
2. synchronized_ostream::operator<< would delegate the printing of "Hello, " to cout
3. operator<<(std::ostream&, const char*) would print "Hello, "
4. synchronized_ostream::operator<< would construct a locked_ostream and pass the lock to that
5. locked_ostream::operator<< would delegate the printing of name to cout
6. operator<<(std::ostream&, std::string) would print the name
7. The same delegation to cout happens for the exclamation point and the endline manipulator
8. The locked_ostream temporary gets destructed, the lock is released

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